1. Field of the Invention
The present invention relates to a focusing method of a charged particle beam and an astigmatism adjusting method of a charged particle beam, for example, a focusing method and an astigmatism adjusting method in an electron beam writing apparatus which irradiates an electron beam onto a target workpiece while variably shaping the electron beam.
2. Related Art
A lithography technique which bears the development of miniaturization of a semiconductor device is a very important process of semiconductor manufacturing processes. In recent years, with high-density integration of an LSI, a line width required for a semiconductor device is decreased every year. A lithography technique to form a desired circuit pattern on the semiconductor device requires an accurate original pattern (also called a reticle or a mask). In this case, an electron beam writing technique essentially has a high resolution and a high accuracy, and is used in production of the accurate original pattern.
FIG. 11 is a conceptual diagram for explaining an operation of a conventional variable-shaping electron beam writing apparatus.
In the variable shaping electron beam (EB) writing apparatus, a pattern will be written as follows. In a first aperture plate 410, an oblong (for example, rectangular) opening 411 to shape an electron beam 330 is formed. In a second aperture plate 420, a variable shaping opening 421 to shape the electron beam 330 passing through the rectangular opening 411 into a desired oblong shape is formed. The electron beam 330 irradiated from a charged particle source 430 and passing through the rectangular opening 411 is deflected by a deflector. The electron beam 330 passes through apart of the variable shaping opening 421 and is irradiated on a target object 340 placed on a stage such that the electron beam 330 is focused by a coil lens or the like. At this time, the stage continuously moves in a predetermined direction (for example, an X direction). More specifically, an oblong shape which can pass through both the opening 411 and the variable shaping opening 421 is written in a writing region of the target object 340. A scheme which causes an electron beam to pass through both the opening 411 and the variable shaping opening 421 to form an arbitrary shape is called a variable shaping scheme. With respect to the variable shaping electron beam writing apparatus, disclosed documents are present (for example, see Published Unexamined Japanese Patent Application No. 9-293670(JP-A-9-293670)).
In the electron beam writing apparatus, a beam must be focused on the target object. An example of the focusing method will be described below.
FIG. 12 is a diagram showing an example of the focusing method.
For example, a calibration mark 504 is independently prepared, an adjusting method which a beam is focused on a position of the calibration mark 504 by using a lens coil 502 is used. However, since a target object plane 510 is not always a plane, a level position of the calibration mark 504 is not equal to a level position of the entire surface of the target object plane 510, and an error generated by the inclination must be corrected. In general, an electromagnetic lens having a dynamic range is used in rough focal point adjustment. A remaining error is separately tried to be with high accuracy corrected at a high speed by using an electrostatic lens. However, with miniaturization of a pattern to be written, it is desired that the error be more accurately corrected. For this purpose, a resolution of the electrostatic lens must be improved. For this purpose, a dynamic range 506 of the electrostatic lens is advantageously narrowed. As shown in FIG. 12, when the entire surface of the target object plane 510 falls within the range of the dynamic range 506 of the electrostatic lens, focus adjustment can be performed by using the electrostatic lens.
FIG. 13 is a diagram showing an example in which the entire surface of the target object plane 510 falls out of the dynamic range of the electrostatic lens.
With the dynamic range 506 narrowed, when a position serving as a reference for focusing a beam is the position of the calibration mark 504, depending on the inclination of the target object plane, as shown in FIG. 13, a part of the target object plane falls out of the dynamic range 506. FIG. 13 shows a state in which the target object plane 512 falls out of the dynamic range 506. When the target object plane 512 falls out of the dynamic range 506, a focal point cannot be corrected. For this reason, it is disadvantageously difficult to write an accurate pattern.
In an electron beam writing apparatus, even though a beam is focused, aberration correction, in particular, astigmatism adjustment must be performed. Even in the astigmatism adjustment, an adjusting method which adjusts an astigmatism at the position of the calibration mark 504 is used. However, as described above, since the target object plane is not always plane, a level position of the calibration mark is not always equal to a level position of the entire surface of the target object plane, and an error generated by the inclination must be corrected. The error is also tried to be accurately corrected by an electrostatic lens which is independently arranged. In this case, as described above, a part of the target object plane may fall out of the dynamic range. When the target object plane falls out of the dynamic range, astigmatism adjustment cannot be performed. For this reason, it is disadvantageously difficult to write an accurate pattern.
With miniaturization of a pattern to be written, a method of solving these problems is desired. However, this proposal provides a method to use a dynamic range of an electrostatic lens as efficiently as possible.